I am trying to understand context in golang. I copied an example from https://golang.org/pkg/context/#example_WithCancel and changed it a bit: Playgroud: https://play.golang.org/p/Aczc2CqcVZR
package main
import (
"context"
"fmt"
"time"
)
func main() {
// gen generates integers in a separate goroutine and
// sends them to the returned channel.
// The callers of gen need to cancel the context once
// they are done consuming generated integers not to leak
// the internal goroutine started by gen.
gen := func(ctx context.Context) <-chan int {
dst := make(chan int)
n := 1
go func() {
for {
select {
case <-ctx.Done():
fmt.Println("DONE")
return // returning not to leak the goroutine
case dst <- n:
n++
}
}
fmt.Println("END")
}()
return dst
}
ctx, cancel := context.WithCancel(context.Background())
defer time.Sleep(1 * time.Second)
defer fmt.Println("Before cancel")
defer cancel() // cancel when we are finished consuming integers
defer fmt.Println("After cancel")
channel := gen(ctx)
for n := range channel {
fmt.Println(n)
if n == 5 {
break
}
}
fmt.Println( <-channel)
}
When commenting out the
defer time.Sleep(1 * time.Second)
the "DONE" never gets printed. Playgroud: (https://play.golang.org/p/K0OcyZaj_xK)
I would expect the go routine which was started in the anonymous function still to be active. Once cancel() is called due to being deferred, the select should no longer block as
case <-ctx.Done():
should be available. However it seems to just end, unless I wait for 1 second and give it time. This behavior seems very wrong.
This behavior seems very wrong.
It's not. That's how program execution is specified. After main and its deferred functions return, the program exits.
Program execution begins by initializing the main package and then invoking the function main. When that function invocation returns, the program exits. It does not wait for other (non-main) goroutines to complete.